The present invention relates to a method for identifying periodically occurring like signals that are part of a signal mixture including interference components, particularly for detecting QRS complexes of fetal heart signals in an abdominally derived fetomaternal electrocardiogram, as well as to an apparatus for practicing the method.
The detection and identification of periodically occurring like signals in a signal mixture containing interference signals represents a goal which has not yet been satisfactorily reached in, for example, perinatalogy where it is desired to detect and process fetal electrocardiograms (EKGs). A fetal EKG cannot be derived directly from the fetus with the aid of scalp electrodes until immediately preceding birth, after the amnionic sac has broken. Since, therefore, this cannot be done during the entire period of pregnancy, methods of indirectly deriving the fetal EKG from the mother must be used if effective monitoring of the fetus is to be achieved. However, electrical signals derived from the mother's abdomen have a very unfavorable signal to noise ratio so that the discovery quota for the occurrence of fetal QRS complexes with the prior art filtering techniques is only about 50%. A fetal QRS complex is a characteristic component of the heart signal, specifically a signal occurrence containing P, Q and R peaks. Since this is sufficient, at most, to indicate the average fetal heart frequency, the physician has available to him neither sufficient data about microfluctuations, i.e., temporary fluctuations in the heart frequency, nor about the wave shape of the fetal EKG.
Interfering with the fetal EKG are the maternal EKG, the maternal electromyogram signals due to muscle movements and noise signals. Their amplitudes are much higher than that of the fetal electrocardiogram whose maximum amplitude when recorded by means of abdominal electrodes is 10 to 50 .mu.V. The amplitude of the fetal EKG changes considerably, however, depending on the position of the fetus with respect to the electrodes.
A maternal abdominal signal has the following composition: EQU S(t)=D(t).multidot.[E.sub.f (t)+E.sub.m (t)+N(t)]
where
S(t)=abdominal signal PA1 D(t)=multiplicative distortion PA1 E.sub.f (t)=fetal EKG PA1 E.sub.m (t)=maternal EKG PA1 N(t)=noise and maternal electromyogram PA1 S(t)=the signal mixture to be processed; PA1 E(t)=the interference signal to be suppressed: PA1 N(t)=the useful signal (fetal EKG) and PA1 E(t)=k.multidot.M(t) where PA1 M(t)=the sample of the interference signals.
The interferences from E.sub.m (t) and N(t) are additively superposed on the fetal EKG. The additional multiplicative distortions originate mainly from movements which occur but their influence is so slight that a good approximation can be reached if they are ignored, in which case the result will be: EQU S(t)=E.sub.f (t)+E.sub.m (t)+N(t).
The wave shapes of the individual components are not known in advance and are often even subject to fluctuations within the time interval in which the signals are being processed so that evaluation is made substantially more difficult.
The maternal QRS complex has its greatest power density in the frequency range between 10 and 30 Hz. The maximum of the fetal QRS complex in the frequency spectrum is a bit higher, i.e., between 15 and 40 Hz.
The noise components contained in the abdominal signal extends over the entire frequency range. Very low frequency noise probably originates mainly from movement of the electrodes, while the higher frequency portion must be attributed to the electromyogram.
In practice, the following methods have been used to detect the fetal electrocardiogram in the abdominal signal: